A flexible, three-axis carbon nanotube (CNT)–polymer composite-based tactile sensor is presented. The proposed sensor consists of a flexible substrate, four sensing cells, and a bump structure. A CNT–polydimethylsiloxane (PDMS) composite is produced by a solvent evaporation method, and thus, the CNTs are well-dispersed within the PDMS matrix. The composite is directly patterned onto a flexible substrate using a screen printing technique to fabricate a sensor with four sensing cells. When a force is applied on the bump, the magnitude and direction of force could be detected by comparing the changes in electrical resistance of each sensing cell caused by the piezoresistive effect of the composite. The experimentally verified sensing characteristics of the fabricated sensor exhibit a linear relationship between the resistance change and the applied force, and the measured sensitivities of the sensor for the normal and shear forces are 6.67 and 86.7%/N for forces up to 2.0 and 0.5 N, respectively. Experiments to verify the load-sensing repeatability show a maximum 2.00% deviation of the resistance change within the tested force range.

A flexible piezoelectric strain energy harvester that is responsive to multi-directional input forces produced by various human motions is proposed. The structure of the harvester, which includes a polydimethylsiloxane (PDMS) bump, facilitates the effective conversion of strain energy, produced by input forces applied in random directions, into electrical energy. The structural design of the PDMS bump and frame as well as the slits in the piezoelectric polyvinylidene fluoride (PVDF) film provide mechanical flexibility and enhance the strain induced in the PVDF film under input forces applied at various angles. The amount and direction of the strain induced in PVDF can be changed by the direction of the applied force; thus, the generated output power can be varied. The measured maximum output peak voltage is 1.75, 1.29, and 0.98 V when an input force of 4 N (2 Hz) is applied at angles of 0°, 45°, and 90°, and the corresponding maximum output power is 0.064, 0.026, and 0.02 μW, respectively. Moreover, the harvester stably generates output voltage over 1.4 × 104 cycles. Thus, the proposed harvester successfully identifies and converts strain energy produced by multi-directional input forces by various human motions into electrical energy. We demonstrate the potential utility of the proposed flexible energy harvester as a self-powered human motion sensor for wireless healthcare systems.

various fabrics [7][8][9][10][11][12] and fibers [13][14][15] for fabricating triboelectric energy harvesters. Fabric-or fiber-based wearable energy harvesters facilitate lightweight energy generation systems that are comfortable for the wearer. [1] However, the output voltage of triboelectric energy harvesters is drastically reduced by adsorbed water molecules, [16] such as humidity originating from the body, rain, or other surrounding environmental conditions. In general, the output voltage of a triboelectric energy harvester is proportional to the charge density of the contact surface. [17] Under high relative humidity conditions, a thick adsorbed water layer increases the conductivity of the contact surface, causing dissipation of surface charges induced by triboelectrification to another material. [18,19] The decreased charge density of the contact surface results in the deterioration of the output voltage; therefore, there is motivation to develop a humidity-resistant, wearable, triboelectric energy harvester. To date, there have been some efforts to impart humidity resistance to triboelectric energy harvesters. [20][21][22][23][24][25][26] For example, Seol et al. reported a triboelectric vibrational energy harvester sealed in an acrylic tube to reduce the penetration of humidity; this device showed excellent resistance to ambient humidity. [20] Additionally, various methods for fabricating humidity-resistant triboelectric energy harvesters have been reported, including a hydrophobic microsponge structure, [21,22] nature-replicating micro-/nanostructure, [23] and micro/nanosurface morphologies [24,25] using polystyrene microbeads and natural materials with micro-/nanomorphologies as a template or mold. However, no further research has been undertaken to apply these techniques to fabric-based, wearable triboelectric energy harvesters.Kim et al. reported a fabric-based, wearable, humidityresistant triboelectric energy harvester by fabricating individual ZnO-polydimethylsiloxane core-shell fibers. [26] Each core-shell fiber was sealed at both sides by a polymer and then woven to prepare a fabric-based triboelectric energy harvester. This harvester showed excellent humidity resistance up to a relative humidity of 95%; however, the fabrication of the humidityresistant fabric is very complicated as it required the formation of nanostructures, fiber-combining, and sealing of each fiber.The development of fabric-based triboelectric energy harvesters is of great interest for converting human motion into electricity and is relevant for the development of wearable electronics. However, such harvesters exhibit significant degradation in performance under high humidity conditions. To solve this problem, a humidity-resistant, fabric-based triboelectric energy harvester by depositing self-assembled monolayers (SAM) to increase the hydrophobicity of the fabric surface is demonstrated. The SAM coating is compatible with various fabrics and a noticeable improvement in triboelectric performance under high humidity conditions...

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Energy harvesting from ambient low-frequency magnetic field using magneto-mechano-electric composite cantilever Appl. Phys. Lett.